The present invention relates to heat-sensitive recording sheets.
The term "heat-sensitive recording sheet" is used herein to describe a sheet which is capable of being used to produce a recorded image by utilizing a physical or chemical change of substances by heat energy, and a large number of processes for heat-sensitive recording sheets have been studied.
Heat-sensitive recording sheets utilizing a physical change of substances by heat include the so-called wax-type heat-sensitive recording sheet which has been used for, for example, electrocardiograms. Further, heat-sensitive recording sheets utilizing a chemical change by heat involve various color formation mechanisms and, particularly, the so-called two-component color heat-sensitive recording sheets are typical examples.
A two-component color heat-sensitive recording sheet is one which is produced by finely dispersing two kinds of heat-reactive compounds, mixing them with a binder and applying the resulting mixture to a support so that the two kinds of heat-reactive compounds are separated from each other by the binder. Recording is carried out by utilizing the color reaction which occurs on melting of one or both of the compounds by applying heat so that they come into contact with each other. These two kinds of heat-reactive compounds are generally an electron donor compound and an electron acceptor compound. Many combinations of these compounds which can be roughly classified as those which form images of metal compounds and those which form dye images, are known.
Examples of combinations forming images of metal compounds are those using organic reducing agents including phenols, etc., chelating agents, sulfur compounds and amino compounds as the electron donor compound and organic metal salts as the electron acceptor compound. Both of them react on heating and form metals, metal complex compounds or metal sulfides, etc., to produce color images. Specific examples of such combinations include combinations of thiourea and heavy metal salts (e.g., as disclosed in U.S. Pat. No. 2,740,895), combinations of gallic acid and metal salts of stearic acid (e.g., as disclosed in U.S. Pat. Nos. 2,663,654-2,663,657), combinations of hydroquinone and silver behenate (e.g., as disclosed in U.S. Pat. No. 3,031,329) and combinations of hexamethylenetetramine and tin compounds (e.g., as disclosed in U.S. Pat. No. 2,813,043), etc.
Examples of combinations forming dye images, include those using electron donor colorless dyes as the electron donor compound and acid substances including phenols as the electron acceptor compound (e.g., as disclosed in Japanese Patent Publication Nos. 4160/68 and 3680/69).
These two component color heat-sensitive recording sheets have a number of advantages in that (1) development is not required because of the primary color formation, (2) the quality of the paper closely resembles that of conventional paper and (3) they can be easily handled. Particularly, where colorless dyes are used as the electron donor compound, such sheets have great utilitarian value because they possess additional advantages in that (4) the color density formed is high and (5) it is easy to produce heat-sensitive recording sheets in which various color hues can be formed. Accordingly, this type is the most widely utilized as heat-sensitive recording elements.
Recently heat-sensitive recording sheets having the above described characteristics have been employed as image receiving recording papers for facsimile communication use.
In using heat-sensitive recording sheets as a facsimile recording paper, the structure of the recording device is simplified because no development step is required, and maintenance advantages arise because supplies other than the recording paper are not required. However, thermal recording has a disadvantage. Namely, the recording rate for thermal recording is low. An important reason for the low rate is believed to be the inferior thermal response of the thermal recording element and the heat-sensitive recording sheet. Although thermal recording elements having a good thermal response have been developed recently, no heat-sensitive recording sheet is capable of sufficiently responding thereto. Accordingly, development of such heat-sensitive recording sheets has been desired.